[1]} made the important discovery that if the apparent brightnesses of an AGN in different passbands – for example, the fluxes \(F_B\)  and \(F_V\)  in the Johnson \(B\)  and \(V\)  bands – are plotted against each other as fluxes rather than as magnitudes, the curvature seen when plotting in magnitudes disappears. He interpreted the resulting straight lines in flux–flux plotsNote that although the standard procedure has been to determine gradients from flux–flux plots, the gradient can also be obtained from root-mean-square spectra. as the addition of a non-varying component (mostly starlight of the host galaxy) and a varying AGN component of fixed spectral shape. He further suggested that differences in the slope in flux-variability plots were a consequence of differing reddening in each AGN. Careful observations and analysis by [2]}, [3]} and [4]} have shown that the loci in optical flux-variability plots are indeed straight lines. [2]}, [6]} and [7]} also show that there is reasonable agreement with other reddening indicators, thus supporting Chołoniewski's conjecture.
